THE FIRST STEP OF GLYCOSYLPHOSPHATIDYLINOSITOL BIOSYNTHESIS IS MEDIATED BY A COMPLEX OF PIG-A, PIG-H, PIG-C AND GPI1

Biosynthesis of glycosylphosphatidylinositol (GPI) is initiated by tra nsfer of N-acetylglucosamine (GlcNAc) from UDP-GlcNAc to phosphatidyli nositol (PI). This chemically simple step is genetically complex becau se three genes are required in both mammals and yeast. Mammalian PIG-A and PIG-C are homologous to yeast GPI3 and GPI2, respectively; howeve r, mammalian PIG-H is not homologous to yeast GPI1. Here, we report cl oning of a human homolog of GPI1 (hGPI1) and demonstrate that four mam malian gene products form a protein complex in the endoplasmic reticul um membrane. PIG-L, which is involved in the second step in GPI synthe sis, GlcNAc-PI de-N-acetylation, did not associate with the isolated c omplex. The protein complex had GPI-GlcNAc transferase (GPI-GnT) activ ity in vitro, but did not mediate the second reaction. Bovine PI was u tilized similar to 100-fold more efficiently than soybean PI as a subs trate, and lyse PI was a very inefficient substrate. These results sug gest that GPI- GnT recognizes the fatty acyl chains of PI. The unusual ly complex organization of GPI-GnT may be relevant to selective usage of PI and/or regulation.